Symevnt are Dynamic Link Libraries and Virtual Device Drivers for Symmantec Event Library.
By default, SYMEVNT.DLL and SYMEVNT1.386 are placed in C:\Program Files\Symmantec and C:\WINDOWS\SYSTEM.

Use safe mode and delete your Symmantec application and reboot.

Make sure you completely remove NAV as it leaves traces scattered throughout your folders. Manually go through each folder in tree view in explorer to be certain.
I think from memory, Symmantic make a program to remove it completely.